[stryker]

I'm going to assume that you're not talking about the secondary market for used goods, because that can most definitely be a business. See: ebay.com.

About arbitrage: I agree with you, to some extent. By definition, as more people notice the price differences and take advantage of them, the arbitrage will become less and less profitable. If you intend to build a business that will scale and prosper in the long term, you need to provide value to people and not tie yourself to short-lived arbitrage opportunities. I work in finance and that fact is plainly obvious to a lot of us.

However, there is value added when you give people information. Information isn't free. Someone has to go out there and find all the prices, organize them, and show them to people. The arbitrage differential will grow smaller and smaller as competition grows, but at no point will this information be free. Given the possibility to find cheaper prices, people will pay if it's worth their while. If it's not money, at least page views.